Electrochemical devices such as zinc/air primary batteries and nickel/hydrogen satellite batteries use barrier materials and/or gas diffusion membranes. Often the required permeability of these films to gases or vapors varies depending on the intended use, and can range from microporous, high flow-rate, membranes to very dense, (almost full density) fluoropolymer membranes (as in air depolarized cells). This range of permeabilities is typically realized by choosing membranes with different pore sizes and/or porosities.
Gas diffusion membranes have been used with gas diffusion electrodes at the anode, cathode, or both. For instance, a nickel-hydrogen battery uses a hydrogen electrode, the zinc-air battery uses an oxygen electrode, and the alkaline fuel cell uses both. The main advantages to using a gas diffusion electrode are that the reagents necessary for the electrochemical reaction can be stored outside the immediate electrochemical cell (as in the case for the hydrogen electrode) or absorbed from the environment (as in the case of the oxygen electrode). In particular, the zinc-air battery has an energy density of about twice that of an alkaline zinc-manganese dioxide battery because only the zinc needs be stored within the battery. Oxygen for the cathode reaction is supplied from the ambient air.
In the manufacturing process of many electrochemical cells, a thin, flexible polymer sheet is processed between rollers on automated or semi-automated equipment. Because this process puts considerable stress on the sheet as it is pulled from the rollers, it is necessary that the sheet has a sufficiently high tensile strength to survive the process.
Additionally, under certain conditions, the electrochemical cell components can change dimensions radically. For example, when a Zn/air cell is exposed to a high relative humidity environment (e.g, >60% RH) it will absorb H2O and swell, creating significant pressure on the air cathode. In this situation, existing hydrophobic gas diffusion membranes may even debond or delaminate from the active layer. This can result in a phenomenon called “flooding”, wherein electrolyte leaks out of the cell or displaces most of the air in the catalytic active layer, thereby reducing the electrical power output.
Extremely aggressive reduction or oxidation reactions are present inside many electrochemical cells. Often the hydrophobic gas diffusion layers must survive years in this environment with no loss of mechanical strength or change in physical structure. Fluoropolymers are a common material of choice since they have excellent chemical resistance.
The intended use of an electrochemical device may delineate the physical and chemical requirements of the gas diffusion membrane or barrier material. For example, batteries designed to be used in devices that could be shaken or subjected to repeated or sudden vibrations must be able to perform even after sustaining repeated mechanical shocks. Humidity and temperature extremes are also a common test for the final packaged electrochemical cell.
A schematic representation of an exemplary prior art zinc/air button cell 100 is given in FIG. 1. The cell 100 includes an anode can 130 having zinc anode material 120 that is mixed with an electrolyte and optionally with a gelling agent. The cathode can 140 includes a separator 110, an air electrode with a catalytically active layer embedded in a metal mesh that acts as a current collector, a bonded hydrophobic membrane, a loose diffusion membrane, and optionally an air distribution layer.
In a known method of fabricating a catalytically active layer, a thin, flexible polymer sheet is processed by nip roll processing and then adding a Ni mesh by paint coating. In another known method, a polymer sheet is formed by paste extrusion and calendered before being laminated to an expanded nickel metal collector. As in the case for the thin polymer sheets mentioned above, the manufacturing process of the catalytically active layer is also performed with automated or semi-automated equipment that stresses the sheet. Thus, the sheet must have a sufficiently high tensile strength to survive this process and further processing. In most commercial applications, the strength is provided by the metal mesh. An additional complication is that, under certain conditions, the electrochemical cell components in contact with the electrolyte can change dimensions radically if just paste extruded and not sintered.
As an example of a prior art device, there is known an air depolarized cell which uses a nickel grid as the metal support for the catalyst. The supported catalyst layer is prepared by mixing manganese dioxide with activated carbon to produce a paste that is applied directly onto the nickel grid. The paste/grid assembly is then dried and bonded to a hydrophobic polymer, such as a fluorocarbon polymer, which serves as a gas diffusion membrane. In an alternative embodiment of this prior invention, the activated carbon is replaced by sintered nickel to improve the conductivity and the efficiency of the current collection. Although this method can be used to produce a serviceable air cathode, it is likely that the reliance on using a metal grid (and optionally sintered nickel in the catalyst) makes the battery expensive and undesirably heavy.
In another prior air depolarized cell, an electrode is pressed into a perforated steel drum that is used to reinforce the electrode and to collect current. A perforated metallic lid is used as the gas diffusion membrane. However, it is known in the art that manufacturing this type of cell is difficult in practice, because the inner surface of the casing neck edge almost invariably becomes fouled as the electrode is pressed into the perforated steel drum.
Other prior air depolarized cells have relied on using a metal wire as the current collector. One example of such a device uses a metal wire that is wound in the shape of a helix and embedded in the catalytically active material. However, a potential drawback of this approach is that the wire only provides one current collection path, so that the current collection efficiency of the cell would be significantly reduced if the wire were to break.
There are also prior art examples of air diffusion membranes. For example, one known type of membrane is made from a relatively thin (0.05-0.15 mm) microporous sheet of polytetrafluoroethylene (PTFE). To reduce the rate at which oxygen arrives at the catalytically active sites, the gas diffusion membrane is wound multiple times around the catalytically active layer. A major drawback of this approach, however, is that the multiple windings of the gas diffusion membrane occupy space that would otherwise be used to store more anode material.
Another prior art membrane is constructed from a porous plate of active carbon that is rendered hydrophobic by the incorporation of polystyrene. As with the previously described membrane, this design suffers from the drawback that the gas diffusion membrane is bulky and occupies space that would otherwise be used to store more anode material.
A further prior art membrane is made from a thin metallic sheet which has small apertures that are formed by laser drilling. Although this approach can be used to fabricate well-defined gas-inlet apertures, large scale manufacturing of this type of gas diffusion membrane is likely to be impractically slow. Furthermore, since the metal membranes do not permit lateral oxygen diffusion, the distribution of oxygen to the catalyst layer is non-uniform.
Another prior art approach involves using a heat shrinkable plastic material to encase the cathode portion of the cell. In order to increase current output for high drain applications, the heat shrinkable plastic material is cut at predetermined points to produce slits that are up to 0.13 cm. wide. For low drain applications, the slit dimensions are reduced. However, the process of putting such macroscopic slits in the gas diffusion membrane very likely leads to grossly non-uniform lateral distribution of oxygen to the catalyst.